ZnO thin films have a direct band gap of 3.23 eV. D. M. Bagnall, et al., Appl. Phys. Lett. 70, 2230 (1997), suggest the use of ZnO thin films in a variety of applications, including optoelectronic devices in the ultraviolet region. ZnO thin films have a high excitonic energy (approximately 60 meV). Moreover, ZnO in solid solution with MgO can produce higher band-gap, MgxZn1-xO (hereinafter “MZO”) alloys.
A. Ohtomo, et al., Appl. Phys. Lett. 77, 975 (2000), suggest the use of MZO alloys for quantum well structures.
T. Mukano, et al., Appl. Phys. Lett. 78, 1979 (2000) reports the quantum well structure of ZnO/MgZnO muiltilayers, explaining the radiative recombination of electron-hole pairs in terms of quantum-confined Stark and Franz-Keldish effects. According to the phase diagram of ZnO—MgO binary systems as explained by E. R. Segnit, et al., J. Am. Ceram. Soc. 48, 412 (1965), the thermodynamic solid solubility of MgO in a ZnO matrix is less than 4 mol %.
The crystal structures of ZnO (which is wurtzite hexagonal, with a=53.24 Å and c=55.20 Å) and MgO (which is cubic, with a=54.24 Å) are entirely different. However, the ionic radii of Mg2+ (which is 0.57 Å) and Zn2+ (which is 0.60 Å) are quite close and may alloy by replacing each other in an MZO matrix. Similarly, the ionic radii of MgO (which is 0.136 nm) and ZnO (which is 0.125) are quite close.
A. Ohtomo, et al., Appl. Phys. Lett. 77, 975 (2000) teach that alloying ZnO with different concentrations of MgO can enhance its band gap. Generally, with higher Mg concentrations, the MZO alloys form a metastable material. The metastable phase and degree of metastability are the limiting factors for practical applications of MZO based devices.
A. Ohotomo, et al., Appl. Phys. Lett. 72, 2466 (1998) teach the formation of highly c-axis oriented, metastable, hexagonal MZO thin films with 30% Mg contents. These MZO thin films were grown by the pulsed laser deposition (PLD) technique on single crystal (0001) Al2O3. According to them, MZO films with a Mg concentration above 33% were segregated to hexagonal and cubic phase.
J. Naryana, et al., Solid State Commun. 121, 9 (2002), teach the growth of epitaxial cubic MZO thin films with a composition of Mg0.8Zn0.2O on Si substrates. These MZO thin films did not have segregation of any secondary phase.
S. Choopun, et al., Appl. Phys. Lett. 80, 1529 (2002) teach the fabrication of a wide-band-gap (approximately 6 eV), metastable MZO alloy. This was obtained with 50% Mg insertion at Zn sites using the PLD technique. Their results showed a wide variation of Mg concentrations (approximately 50%–85%) with the variation in substrate temperature. However, the main drawback of their results was the phase separation of MgO (which is cubic) from ZnO (which is hexagonal) after rapid thermal annealing for 1 min at 750° C. This phase separation reduced the band gap to 3 eV.
Further background is provided in:                (a) Y. R. Ryu, et al., J. Cryst. Growth 219, 419 (2000);        (b) T. Aoki, et al., Appl. Phys. Lett. 76, 3257 (2000);        (c) A. K. Sharma, et al., Appl. Phys. Lett. 75, 3327 (1999);        (d) T. Makino, et al., Appl. Phys. Lett. 81, 2355 (2002);        (e) R. D. Shannon, et al., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 32, 751 (1976); and        (f) J. Sigman, D. P. Norton, H. M. Christen, P. H. Fleming, and L. A. Boatner, Phys. Rev. Lett. 88, 097601 (2002).Each of the above publications, along with each of the publications previously mentioned, is expressly incorporated herein by reference in their entirety.        
In view of the forgoing, a method is desired to fabricate a stable, wide-band-gap MZO alloy. The MZO alloy must not exhibit a significant change in structural and optical properties even after annealing.